On Friday 22 April 2016 07:46 PM, Peter Zijlstra wrote:
> On Fri, Apr 22, 2016 at 10:50:41AM +0000, Vineet Gupta wrote:
>
>>> > > +#define ATOMIC_FETCH_OP(op, c_op, asm_op) \
>>> > > +static inline int atomic_fetch_##op(int i, atomic_t *v) \
>>> > > +{ \
>>> > > + unsigned int val, result; \
>>> > > + SCOND_FAIL_RETRY_VAR_DEF \
>>> > > + \
>>> > > + /* \
>>> > > + * Explicit full memory barrier needed before/after as \
>>> > > + * LLOCK/SCOND thmeselves don't provide any such semantics \
>>> > > + */ \
>>> > > + smp_mb(); \
>>> > > + \
>>> > > + __asm__ __volatile__( \
>>> > > + "1: llock %[val], [%[ctr]] \n" \
>>> > > + " mov %[result], %[val] \n" \
>> >
>> > Calling it result could be a bit confusing, this is meant to be the "orig" value.
>> > So it indeed "result" of the API, but for atomic operation it is pristine value.
>> >
>> > Also we can optimize away that MOV - given there are plenty of regs, so
>> >
>>> > > + " " #asm_op " %[val], %[val], %[i] \n" \
>>> > > + " scond %[val], [%[ctr]] \n" \
>> >
>> > Instead have
>> >
>> > + " " #asm_op " %[result], %[val], %[i] \n" \
>> > + " scond %[result], [%[ctr]] \n" \
>> >
>> >
> Indeed, how about something like so?
>
> ---
> Subject: locking,arc: Implement atomic_fetch_{add,sub,and,andnot,or,xor}()
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Mon Apr 18 01:16:09 CEST 2016
>
> Implement FETCH-OP atomic primitives, these are very similar to the
> existing OP-RETURN primitives we already have, except they return the
> value of the atomic variable _before_ modification.
>
> This is especially useful for irreversible operations -- such as
> bitops (because it becomes impossible to reconstruct the state prior
> to modification).
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
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